Blood vessel occlusions are commonly treated by mechanically enhancing blood flow in the affected vessels, such as by employing a stent. Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of affected vessels. Typically stents are capable of being compressed, so that they can be inserted through small lumens via catheters, and then expanded to a larger diameter once they are at the desired location. Examples in the patent literature disclosing stents include U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor.
FIG. 1 illustrates a conventional stent 10 formed from a plurality of struts 12. The plurality of struts 12 are radially expandable and interconnected by connecting elements 14 that are disposed between adjacent struts 12, leaving lateral openings or gaps 16 between adjacent struts 12. The struts 12 and the connecting elements 14 define a tubular stent body having an outer, tissue-contacting surface and an inner surface.
Stents are being modified to provide drug delivery capabilities. A polymeric carrier, impregnated with a drug or therapeutic substance is coated on a stent. The conventional method of coating is by, for example, applying a composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer. The dipping or spraying of the composition onto the stent can result in a complete coverage of all stent surfaces, i.e., both luminal (inner) and abluminal (outer) surfaces, with a coating. However, having a coating on the luminal surface of the stent can have a detrimental impact on the stent's deliverability as well as the coating's mechanical integrity. Moreover, from a therapeutic standpoint, the therapeutic agents on an inner surface of the stent get washed away by the blood flow and typically can provide for an insignificant therapeutic effect. In contrast, the agents on the outer surfaces of the stent are in contact with the lumen, and provide for the delivery of the agent directly to the tissues. Polymers of a stent coating also elicit a response from the body. Reducing the amount to foreign material can only be beneficial.
Briefly, an inflatable balloon of a catheter assembly is inserted into a hollow bore of a coated stent. The stent is securely mounted on the balloon by a crimping process. The balloon is inflated to implant the stent, deflated, and then withdrawn out from the bore of the stent. A polymeric coating on the inner surface of the stent can increase the coefficient of friction between the stent and the balloon of a catheter assembly on which the stent is crimped for delivery. Additionally, some polymers have a “sticky” or “tacky” consistency. If the polymeric material either increases the coefficient of friction or adherers to the catheter balloon, the effective release of the stent from the balloon after deflation can be compromised. If the stent coating adheres to the balloon, the coating, or parts thereof, can be pulled off the stent during the process of deflation and withdrawal of the balloon following the placement of the stent. Adhesive, polymeric stent coatings can also experience extensive balloon sheer damage post-deployment, which could result in a thrombogenic stent surface and possible embolic debris. The stent coating can stretch when the balloon is expanded and may delaminate as a result of such shear stress.
Another shortcoming of the spray coating and immersion methods is that these methods tend to form defects on stents, such as webbing between adjacent stent struts 12 and connecting elements 14 and the pooling or clumping of coating on the struts 12 and/or connecting elements 14. In addition, spray coating can cause coating defects at the interface between a stent mandrel and the stent 10 as spray coating will coat both the stent 10 and the stent mandrel at this interface, possibly forming a clump. During removal of the stent 10 from the stent mandrel, this clump may detach from the stent 10, thereby leaving an uncoated surface on the stent 10. Alternatively, the clump may remain on the stent 10, thereby yielding a stent 10 with excessive coating.
Another shortcoming of the spray coating method is that a nozzle in a spray coating apparatus can get clogged with particulate when some of the coating substance solidifies. This clogging can deflect or block the spray, thereby yielding an unsatisfactory coating on the stent 10. The need to unclog a nozzle can cause long periods of downtime for a spray coating apparatus, thereby lowering production rates of stents.
Accordingly, a new apparatus and method are needed to enable selective coating of stent surfaces while minimizing the formation of defects and coating apparatus downtime.